Implants have found use in modern medical technology in many different embodiments. They are used, for example, for supporting vessels, hollow organs, and duct systems (endovascular implants, for example stents), for attaching and temporarily fixing tissue implants and tissue transplants, as well as for orthopedic purposes, for example as pins, plates, or screws. One form of implant used particularly often is the stent.
The implantation of stents has become established as one of the most effective therapeutic measures in the treatment of vascular diseases. Stents perform a support function in hollow organs of a patient. For this purpose, some stents have a filigreed support structure made of metallic braces, which are initially in a compressed form for insertion into the body, and are then expanded at the site of application. One of the main fields of application of such stents is to permanently or temporarily widen and keep open vascular constrictions, in particular constrictions (stenoses) of the coronary vessels. In addition, aneurysm stents, for example, are also known which are primarily used for closing off the aneurysm. The support function is also provided.
Some stents have a circumferential wall of sufficient load capacity to keep the constricted vessel open to the desired extent, and have a tubular base body through which blood flows through unhindered. The circumferential wall is generally formed by a lattice-like support structure which allows the stent to be inserted in a compressed state, with a small outer diameter, up to the constriction to be treated in the particular vessel, and at that location, for example by use of a balloon catheter, to be expanded until the vessel has the desired enlarged inner diameter. Alternatively, shape memory materials such as nitinol have the capability for self-expansion when a restoring force is discontinued, thus maintaining a small diameter of the implant. The restoring force is generally exerted on this material by a protective tube.
The implant, in particular the stent, has a base body made of an implant material. An implant material is a nonliving material which is used for medical applications and interacts with biological systems. The basic requirement for use of a material as an implant material, which when properly used is in contact with the bodily surroundings, is compatibility with the body (biocompatibility). Biocompatibility is understood to mean the ability of a material to induce an appropriate tissue reaction in a specific application. This includes adaptation of the chemical, physical, biological, and morphological surface characteristics of an implant to the recipient tissue, with the objective of a clinically sought interaction. The biocompatibility of the implant material is also dependent on the time sequence of the reaction of the biosystem which has received the implant. Relatively short-term irritation and inflammation occur which may result in changes in the tissue. Accordingly, biological systems react in various ways, depending on the characteristics of the implant material. The implant materials may be divided into bioactive, bioinert, and degradable/absorbable materials, depending on the reaction of the biosystem.
Implant materials include polymers, metallic materials, and ceramic materials (as a coating, for example). Biocompatible metals and metal alloys for permanent implants contain, for example, stainless steels (316L, for example), cobalt-based alloys (CoCrMo cast alloys, CoCrMo forged alloys, CoCrWNi forged alloys, and CoCrNiMo forged alloys, for example), pure titanium and titanium alloys (CP titanium, TiAl6V4, or TiAl6Nb7, for example), and gold alloys. For biocorrodible stents, the use of magnesium or pure iron, is known.
It is also known that a higher degree of biocompatibility may be achieved when implant materials are provided with coatings of materials which are compatible with tissue in particular. These materials are usually of an organic or synthetic polymeric nature, and sometimes are of natural origin.
The use of biocorrodible magnesium alloys for temporary implants having filigreed structures is made difficult in particular due to the fact that the implant degrades very rapidly in vivo. Various approaches have been discussed for reducing the corrosion rate, i.e., the speed of degradation. On the one hand, attempts have been made to retard the degradation with respect to the implant by development of appropriate alloys. On the other hand, coatings may be designed to temporarily inhibit the degradation. The latter approach thus requires that the coating prevent or inhibit the corrosion for a reproducible period of time, but after that time quickly allows complete disintegration of the implant.